1. Field of the Invention
The present invention relates to a power-controlling device for a laser diode. More particularly, the present invention relates to a method and apparatus for controlling the power of laser diodes in a laser scanning unit.
2. Description of the Related Art
In general, laser printers are devices for printing images on a sheet of printing paper by imaging laser beams emitted from a laser diode in response to video signals of an input image and transferring a latent image formed on a photosensitive drum to the sheet of printing paper.
FIG. 1 is a block diagram of a conventional apparatus for controlling the power of laser diodes. Referring to FIG. 1, the apparatus comprises first and second optical power controllers 110 and 111, first and second laser diode drivers 120 and 121, a laser scanning unit 130, which includes first and second laser diodes 131 and 133 and first and second light receivers 132 and 134, first and second output voltage sensors 140 and 141, and a printer controller 100.
The printer controller 100 preferably provides first and second reference voltages V_ref1 and V_ref2 for driving the first and second laser diodes 131 and 133. More specifically, the optical power of each of the first and second laser diodes 131 and 133 varies depending on the temperature of the laser scanning unit 130. Therefore, in order to provide the first and second laser diodes 131 and 133 with a desired intensity of recording power under any circumstances, that is, regardless of temperature variations, the first and second reference voltages V_ref1 and V_ref2 applied to the laser diodes 131 should be adjusted according to the temperature of the laser scanning unit 130.
The first optical power controller 110 receives a first control voltage, which approximates the first reference voltage V_ref1, by subtracting an output voltage of one of the first and second laser diodes 131 and 133 from the first reference voltage V_ref1 during a non-imaging period. The first optical power controller 110 performs a proportional-integral (PI) control on a subtraction result, such as an error voltage, to obtain the first control voltage. Likewise, the second optical power controller 111 obtains a second control voltage, which approximates the second reference voltage V_ref2, by subtracting the output voltage of the other of the first and second laser diodes 131 and 133 from the second reference voltage V_ref2 during the non-image period. As above, the second optical power controller 111 performs a proportional-integral (PI) control on a subtraction result, such as an error voltage, to obtain the second control voltage. Thereafter, the first and second optical power controllers 110 and 111 output the first and second control voltages, respectively, to the laser diode drivers 120 and 121, respectively.
The first and second laser diode drivers 120 and 121 drive the first and second laser diodes 131 and 133 using the first and second control voltages received from the optical power controllers 110 and 111, respectively.
The first and second light receivers 132 and 134_sense output currents of the laser diodes 131 and output sensing results to the first and second output voltage sensors 140 and 141, respectively.
In a case where the conventional apparatus for controlling the power of laser diodes includes two or more laser diodes, it needs as many optical power controllers and peripheral circuits (such as, light receivers and output voltage sensors) as the laser diodes. Therefore, as the number of laser diodes increases, the size of an entire board on which the conventional apparatus for controlling the power of laser diodes is realized increases, as does power consumption. In addition, since a reference quantity of light has a fixed value, the conventional apparatus for controlling the power of laser diodes cannot appropriately respond to various environmental factors, such as the temperature of the laser scanning unit 130 or the temperature and humidity inside a laser printer.